Holstein Friesian cattle of known BoLA-A genotype were used to determine whether resistance or susceptibility to persistent lymphocytosis (PL) caused by the bovine leukemia virus is more closely associated with class II genes of the BoLA complex than with BoLA-A. Polymorphisms of BoLA-DQB and -DRB2 genes were identified using restriction fragment length polymorphism (RFLP) analysis. All carriers of DRB2*2A were found to be resistant to PL (P $<$ 0.005), whereas DRB2*1C homozygotes were at higher risk for the development of PL (P $<$ 0.05). There was a closer association of PL with DRB2 than with DQB or BoLA-A.A polymerase chain reaction (PCR)-RFLP method was developed to identify polymorphism in the hypervariable second exon of the BoLA-DRB3 gene. Thirty-five alleles of the DRB3 gene were defined by PCR-RFLP, five of which are provisional. Studies of associations between disease and/or production traits with alleles of the BoLA complex could be advanced by PCR-RFLP analysis. PCR-RFLP is an excellent tool to study the genetic diversity and evolution of the Bovidae.A silent nucleotide substitution in the second exon of the BoLA-DYA gene was revealed. This polymorphism was used to determine the order and recombination frequencies ($\theta$) between the DRB3, DYA and prolactin (PRL) genes by single sperm typing in concert with primer extension preamplification (PEP), a method for "whole genome amplification". The order PRL - DRB3 - DYA was established as most likely, with $\theta$ = 0.025 and 0.150, respectively.The segregation of BoLA-A, DRB3 and PRL alleles in four paternal half-sib families was investigated by serology and PCR-RFLP. Six recombinants were observed between DRB3 and PRL, but no recombinants between BoLA-A and DRB3 or PRL were identified. As a consequence, the order between BoLA-A and DRB3 relative to PRL could not be determined by $\theta$ between BoLA-A and DRB3 is less than 0.024 (P $<$ 0.05). The estimate for $\theta$ between DRB3 and PRL and 0.047. The availability of DNA from offspring which inherited paternal DRB3 PRL recombinant haplotypes might be useful for mapping other closely linked genes on bovine chromosome 23 on either side of DRB3 or PRL.